The present invention generally relates to composite grafts that have a generally porous graft body and a generally non-porous membrane. The composite graft is of the non-braided and non-woven type, its body has a porosity that provides an environment which is conducive to tissue ingrowth thereinto, and the membrane of the composite graft is a substantially non-porous cylinder. The graft body is formed from a fiber-forming polymer which is extruded into fibers that are wound or spun onto a mandrel, while the membrane is a biocompatible elastomeric polymer sheath that is formed in place on the mandrel, typically being formed on a partially spun vascular graft body. Formation of the membrane is carried out by a procedure that includes directing a polymeric membrane-forming composition toward the mandrel. Typically, the remainder of the graft body will thereafter be wound over the formed membrane in order to provide a composite graft having a porous graft body with a substantially non-porous cylindrical membrane embedded therewithin to thereby provide a composite graft that is porous to promote ingrowth but that has essentially leak-proof sidewalls.
Graft products such as vascular grafts are known to be made by methods which include winding extruded material onto a mandrel in an attempt to provide a degree of porosity that is desired for implantable grafts, especially including providing an ingrowth environment that is particularly suitable for promoting tissue ingrowth at the implantation locations. One such approach is detailed in U.S. Pat. No. 4,475,972, the disclosure of which is incorporated by reference hereinto. That patent describes non-woven vascular grafts that are made by extruding a polyurethane solution under pressure through an opening and then drawing the extruded material while winding same on the mandrel.
While the porosity that is provided by these types of vascular graft structures is extremely desirable and advantageous, a porous vascular graft substrate or wall is, prior to implantation, typically also porous to the extent that blood or other fluids can pass through the porous wall or porous substrate. Typically, before such a porous implantable device is implanted, the surgical team must subject it to preclotting with blood or the like in order to prevent fluid loss upon initial pressurization of the vascular graft. Such a preclotting step is considered to be a time-consuming annoyance and can provide a potential opportunity for the development of thrombosis and/or infection. Also, the preclotting procedure can vary the environment provided by the vascular graft depending upon the manner in which the techniques are employed by the individuals carrying out the preclotting procedure, which can affect the reproducibility of vascular graft implantation techniques.
The problem of attempting to eliminate the need for preclotting of vascular grafts is rendered more difficult because, for many vascular grafts, it is important to provide certain porosity properties for both internal and external surfaces thereof. In the example of a tubular vascular graft, its external cylindrical surface can be rendered porous in order to provide a porous depth that affords a means for fixation to host tissues by soft tissue ingrowth into the porous depth of the surface, while the internal generally cylindrical porous surface affords a porous interface having smaller pore sizes which provide tissue-implant interfaces that are blood compatible arising from colonization and tissue formation on the blood-contacting internal surface of such vascular grafts. In such instances, tissue ingrowth is desired on the external surface, while endothelial-like cell ingrowth provided by nucleated blood cells or the like is desired on the internal surface. For these reasons, it may be most desirable to provide a vascular graft that is porous on both of its surfaces, but that still prevents fluid passage from one surface to the other without requiring a preclotting operation or the like.
These aspects which are associated with porous vascular grafts in general are taken into consideration by the present invention, which, in summary, includes a substantially non-porous membrane that is formed in place and in association with the spinning and winding procedure that is used to form the graft. More particularly, the present invention is a composite graft that includes a generally porous graft body which is formed by extruding a fiber-forming polymer in association with a rotating mandrel and that further includes a generally non-porous membrane that is formed in place within, under or over the body and also in association with the rotating mandrel. In one embodiment, this membrane formation includes an electrostatically assisted deposition of a biocompatible elastomeric polymer. In its preferred form, the composite graft is made by first partially forming the graft body, suitably depositing the elastomeric polymer and forming the membrane, and then completing the winding of the graft body. A preferred membrane-forming biocompatible elastomeric polymer is a silicone rubber material.
It is accordingly a general object of the present invention to provide an improved non-woven graft.
Another object of this invention is to provide an improved graft having a porous surface for promoting ingrowth and a non-porous membrane.
Another object of the present invention is to provide an improved graft and method for making same that has a porous ingrowth surface but does not permit the passage of fluids therethrough.
Another object of this invention is to provide an improved porous graft and method of making same which does not require preclotting.
These and other objects, features and advantages of this invention will be clearly understood through a consideration of the following detailed description.